Many modern capacitors are made using a capacitor body with metallized film. The metallized film typically includes a thin polymer film such as polypropylene on which a thin metal film has been condensed or otherwise deposited. The metallized film is arranged in a manner in which the thin metal films form two separate internal electrodes that are separated by the polymer film. The two separate internal electrodes are substantially electrically isolated from one another and a capacitance is exhibited between the internal electrodes. The thin metal film of each internal electrode is connected to an end electrode and terminations are connected to each end electrode to electrically connect to the capacitor.
In some instances, two separate sheets of metallized film are rolled or wound together into a cylindrical shaped capacitor body having two generally circular ends. The sheets of metallized film are offset from one another so that each separate sheet only extends all the way to one of the generally circular ends. In these instances, each end electrode is positioned at one of the generally circular ends and is connected to the sheet of metallized film that extends to the end where the end electrode is positioned. This construction yields an annular form capacitor that has a cross-section with layers of metallized film that alternate between the two separate internal electrodes formed by the thin metal film of each sheet. In this instance, each of the end electrodes connects together the layers of the respective internal electrodes.
In other instances, each of the internal electrodes is formed of separate layers of the metallized film to create a rectangular shaped capacitor body. In these instances the layers of one internal electrode are arranged to alternate with the layers of the other internal electrode and are offset from one another on two ends. The end electrodes in these instances, provide an electrical connection between the individual layers of each internal electrode at the offset ends. Other types of capacitors have layers of metal and dielectric such as polymers, in various arrangements, which are not affixed to one another prior to the assembly of the capacitor.
One common technique that is used to create the end electrodes is called end spray. In this technique a molten end spray metal, which may include tin, zinc or other conductive materials, is sprayed onto each of the offset ends of the layers of metallized film. The spray continues until the end spray metal builds up to a certain thickness. The end spray metal sticks to the metallized film and, when the molten metal cools and solidifies, the end spray metal is electrically connected to the metal of the metallized film. The solidified end spray metal on each end connects to one of the internal electrodes where they serve as the end electrodes. Typically, the end spray metal is sprayed onto the ends of the metallized film in as uniform a manner as possible. Other techniques for creating metal end electrodes may also be available.
One of the problems encountered in the use of metal or other types of rigid end electrodes relates to the failure of the capacitor due to one or both of the end electrodes cracking. Many cracking problems are thermally induced as a result of the capacitor being subjected to temperature changes and repeated temperature cycling. In some instances, thermally induced cracking includes the condition where one or both end electrodes at least partially separate from the internal electrodes of the capacitor body. Such a separation changes the characteristics of the capacitor, such as the capacitance and/or current carrying or other characteristics of the capacitor. Other types of thermally induced cracking or damage involve the end electrodes themselves cracking into pieces.
Operation of the capacitor in environments where wide ranges of temperatures are encountered exacerbates the problems with thermally induced cracking. Some environments, which include some types of tests, subject the capacitor to temperatures ranging from −50° C. to 100° C. When the capacitor is heated, through external and/or internal influences, the layers and end electrodes of the capacitor expand; and when the capacitor is cooled, the layers and end electrodes contract.
In many instances, the cracks are a direct result of the dielectric having a different Coefficient of Thermal Expansion (CTE) than the CTE of the end electrode. In some instances, the CTE of the dielectric is an order of magnitude greater than the CTE of the end electrode. When the CTE of the dielectric is greater than the CTE of the end electrode, the dielectric expands at a greater rate than does the end electrode. This can cause the internal electrodes surrounded by the dielectric to be pulled away from the end electrodes.
Typically capacitors are subject to heat generated externally to the capacitor and internal to the capacitor. External heating comes from the devices and atmosphere surrounding the capacitor which typically causes a more or less uniform heating of the capacitor. On the other hand, internal self heating is caused by electrical losses inside the capacitor. Internal self heating can cause the metallized film or other internal electrode and dielectric to be subjected to a higher temperature than the end electrodes. This situation leads to more rapid failure of the capacitor since the increased temperature experienced by the metallized film causes the metallized film to expand more than the end electrodes, thereby typically causing the end electrodes to break or causing other damage to the capacitor.
In some circumstances, some portions of the inside of the capacitor become hotter than other portions of the inside of the capacitor. In these situations, the metallized film expands more in the hotter portions than in the cooler portions which causes a non-uniform stress on the end electrodes which can lead to capacitor failure.
The rigid end electrodes of some capacitors can be very large, sometimes exceeding ten or more inches in diameter for a cylindrically shaped capacitor, for instance. These large capacitors have many layers of dielectric material which, when heated, expand together to increase the overall dimensions in one or more directions of the capacitor. The overall expansion of each dimension of the dielectric is greater than the overall expansion of each corresponding dimension of the rigid end electrodes. Therefore, since the dielectric and rigid end electrodes are connected to one another but the dielectric is expanding faster than the end electrodes, the end electrodes crack. Thermally induced cracking happens to small capacitors as well as large capacitors, although thermally induced cracking may be more pronounced in large capacitors.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon reading of the specification and a study of the drawings.